What is said of some persons not being able to distinguish colours, I
believe. It may proceed from general weakness, which will render the
differences imperceptible, just as the dusk or twilight makes all colours
one. This defect is most usual in the blue ray, the negative pole.
* * * * *
I conjecture that when finer experiments have been applied, the red,
yellow, and orange rays will be found as capable of communicating magnetic
action as the other rays, though, perhaps, under different circumstances.
Remember this, if you are alive twenty years hence, and think of me.
_July_ 21. 1827.